Typically trans-impedance amplifiers with fixed gain are used in an interferometric fiber-optic gyro (IFOG) system. The fixed gain trans-impedance amplifier is calibrated to maximize voltage out based on power measured at a photodiode. The amplifier bandwidth is also calibrated when the gain is calibrated. The amplifier bandwidth and gain would remain constant over the life of the system. This works well in an IFOG system that experiences very little environmental change. However, during radiation exposure, the optical fiber in the IFOG system darkens and signal strength at the photodiode drops significantly. The fixed gain trans-impedance amplifier provides degraded gyro bias performance until the fiber recovers. Also, degraded gyro bias performance of the fixed gain trans-impedance amplifier can occur due to optical losses increasing over time due to component aging.
Also, it is more costly to use fixed gain trans-impedance amplifiers because they require calibration during assembly, and it would require one to stock a large number of different calibration parts.
Previous variable gain approaches degrade the amplifier bandwidth and would adversely affect the gyro bias performance. See FIGS. 1-3.
Therefore, there exists a need for a variable gain trans-impedance amplifier that can effectively respond to radiation events, unit to unit loss variation, and optical loss associated with end of life, without significantly degrading the amplifier bandwidth.